In the equipment used to indirectly transfer heat between one fluid and another, one of the most common ways to increase the amount of heat which can be transferred is to provide for an increased surface of the material which separates the two fluids and through which the heat is conducted. This is no problem if the heat transfer surface is more or less flat because it is a simple matter to weld fins on either or both sides of the flat surface to increase the amount of heat which can be transferred between fluids on either side of it.
In a situation where a tube is used to transfer heat between a fluid inside of it and a fluid in contact with the outer surface of the tube, it is relatively simple to provide an extended heat transfer surface on the exterior of the tubes. All that is necessary is to weld elements such as fins, channels, strips, studs, spines or other elements to the outside of the tubes.
Providing an extended heat transfer surface in the interior of the tube is not such a simple matter. If the tube is of any appreciable length, it is not possible to position welding equipment inside of the tube to secure the heat transfer elements to the tube unless the tube is of extremely large diameter. As a result, the heat transfer capability of any tube is limited and in many applications the only way to increase the heat transfer capacity of a heat exchanger using tubes is to increase the number of tubes which are used to transfer heat. The result is an increase in cost and size of the heat exchanger.